The present invention relates in general to communication systems, and is particularly directed to a mechanism for enabling a continuous wideband waveform, such as an ultrawideband (UWB) radar signal, to be transmitted without mutual interference with other communication signals that occupy portions of a spectrum containing discontinuous unoccupied spectral regions whose total available bandwidth is at least equal to that of the continuous wideband waveform.
With continuing advances in communication systems, components, modulation techniques and waveforms therefor, coupled with the ongoing demand for more and more bandwidth, there are occasionally requirements to transmit a waveform whose bandwidth is considerably wider than any available contiguous unoccupied portion of the available electromagnetic spectrum. As a non-limiting example, it may be desired to transmit a radar waveform, such as a very wideband (e.g., 500 MHz) chirp signal, using relatively long wavelengths that may be employed by a variety of conventional communication systems, such as broadcast FM and television transmitters, UHF and VHF, cellular radios, and the like.
These longer wavelengths provide such a radar signal with foliage and cultural feature penetration properties not possessed by radar signals having very short wavelengths. Still, for a reasonably acceptable spatial resolution of the imaged target, the bandwidth of the radar signal must be very wide, as resolution can be effectively defined in terms of half the inverse of the signal bandwidth. For example, a 500 MHz wide radar signal can provide a spatial resolution on the order of one foot. Unfortunately, such a bandwidth is considerably wider than any continuous portion of the longer wavelength regions of the regulated electromagnetic spectrum. Namely, such a radar signal won""t xe2x80x98fitxe2x80x99 in the spectral region of interest.
This may be understood by reference to FIG. 1, which is a spectral diagram showing the typical xe2x80x98punctuatedxe2x80x99 occupancy of a 1.0 GHz wide portion of the communication bandwidth (from 0 Hz to 1.0 GHz) by a variety of communication systems users in the vicinity of Washington, D.C. A cursory examination of FIG. 1 reveals that there is simply no unoccupied or potentially available xe2x80x98gapxe2x80x99 between adjacent users that provides room for a relatively wide bandwidth (e.g., 500 MHz) waveform. Indeed, in FIG. 1, the widest gap appears to be on the order of only of 70 MHz.
In accordance with the present invention, this problem is successfully addressed by employing identity transform filters (such as sin(x)/x filters) to coherently fragment or subdivide the frequency continuum of a wideband waveform (such as an ultra wideband radar signal) into a plurality of (variously narrow and wide) spectral segments or sub-band components, that are capable of fitting into unoccupied spectral regions of a partially occupied electromagnetic spectrum, the total useable bandwidth of the unoccupied regions being at least equal to that of the wideband waveform.
A sin(x)/x filter is a preferred identity transform filter due to its simplicity and power. Even though an infinite bank of such filters would be mathematically required to produce an xe2x80x98exactxe2x80x99 identity transform, in practice only a relatively small number of filters is required, especially where the signal (such as a chirp) is inherently band-limited. A sin(x)/x filter bank is closely related to the Fourier transform, differing primarily in integration limits. Attractive properties of the sin(x)/x filter include the fact that it is a simple moving average and that an inverse transformation is obtained by summing the outputs of the filter bank. Additionally, the orthogonality of all of the filters in a filter bank allows independent shaping of the composite filter bank response at N points, where N is the number of sin(x)/x filters.
The fragmented portions of the signal are then independently up- or down-converted to transmission carrier frequencies that will selectively place the translated fragments within such unoccupied portions of the spectrum. At the receiver, the process is reversed, and correction is made for any relative motion (as by using the Lorentz transformation (or an acceptable approximation)). In this manner, the original spectrum can be coherently reconstructed and subsequently processed in a conventional manner.
As will be described, the present invention provides a number of significant features, including the ability of a UWB radar to transparently employ almost any traditional desirable radar signal (such as a chirp) as the basic waveform. The coherent fragmentation mechanism of the present invention allows this basic waveform to be partitioned into available spectrum locations, irrespective of their location in frequency, individual bandwidths, or any intervening band gaps. Dynamic re-allocation of individual transmission frequencies and bandwidths may also be employed, as long as the total spectrum bandwidth available equals or exceeds that of the base UWB signal. At the receiver, only relatively modest processing is required to coherently reconstruct the basic radar waveform from the fragmented spectrum, thus allowing the use of traditional radar waveform processing.
The invention permits use of almost any traditional radar waveform, including single-pulse waveforms. UWB bandwidths on the order of from 60 to 500 MHz can readily be accommodated, as well as operating frequencies from 20 to 600 MHz. Arbitrary PRF and duty cycle are automatically included through a user-selected base waveform (such as chirp). The fraction of bandwidth occupied can range from less than 10% to 100%. There are no fundamental restrictions on spectrum segment widths or spacing. The technique described herein allows daily, hourly, or millisecond scale changes in spectrum usage and base waveform choice.